An Auto Repeat reQuest (ARQ) or Hybrid Auto Repeat reQuest (HARQ) technique is frequently used to improve throughput to perform smooth communication in wireless communication systems.
The ARQ or HARQ technique allows the receiving side to feed presence or absence of received data back to the transmitting side, thereby allowing the transmitting side to retransmit the data when the received data contains an error. In the ARQ or HARQ technique, the receiving side transmits an acknowledgement signal (ACK) to the transmitting side when it has received data transmitted from the transmitting side without error. On the other hand, the receiving side transmits a negative acknowledgement signal (NACK) to the transmitting side when the received data contains an error, thereby allowing the transmitting side to retransmit the data so that the receiving side accurately receives the data.
FIG. 1 illustrates a Hybrid ARQ (HARQ) method in a conventional WCDMA system. In FIG. 1, a base station determines a terminal (or user equipment) which is to receive a packet and the formats (such as coding rate, modulation method, and data amount) of the packet to be transmitted to the terminal. The base station first provides information regarding the determination to the terminal through a downlink control channel (HS-SCCH) and then transmits an HS-DSCH data packet at its associated time. The terminal receives the downlink control channel and reads the formats of the packet to be transmitted to the terminal and the time at which the data is to be transmitted. The terminal can receive the packet using the read information.
After receiving the packet, the terminal decodes the received packet. If the received packet is successfully decoded, the terminal transmits an ACK signal to the base station. Upon receiving the ACK signal, the base station determines that the packet has been successfully transmitted to the terminal and thus can perform a next packet transmission process. If decoding of the received packet fails, the terminal transmits a NACK signal to the base station. Upon receiving the NACK signal, the base station determines that the packet has failed to be transmitted to the terminal and then constructs and retransmits the same data in the same packet format or a new packet format at an appropriate time. Upon receiving the retransmitted data, the terminal attempts to decode the data by combining it with the previously received packet, which it failed to decode, using a variety of methods.
In the HARQ or ARQ technique, the transmitting side performs packet retransmission based on the ACK/NACK feedback from the receiving side as described above. That is, the transmitting side performs packet retransmission if it has received a NACK from the receiving side. On the other hand, the transmitting side does not perform packet retransmission if it has received an ACK from the receiving side. Here, if a buffer in the transmitting side contains remaining data to be sent to the receiving side and an HARQ scheduler has allocated radio resources for transmitting the data, the transmitting side starts transmitting a new packet. In the following description, the term “retransmission packet” refers to a packet which the transmitting side retransmits after receiving a NACK from the receiving side and the term “new transmission packet” refers to the first packet which the transmitting side transmits to the receiving side when starting new packet transmission, which occurs when the transmitting side initiates data transmission to the receiving side or when an ACK is received from the receiving side during data transmission. That is, new transmission packets are packets which are not retransmission packets.
In the ARQ or HARQ technique, it is very important that the receiving side accurately feed ACKs/NACKs back to the transmitting side. However, it is likely that an error will occur during transmission of ACK/NACK data, unlike transmission of other upper channel data, because the ACK/NACK data consists of 1 or 2 bits and also because it is not easy to apply any additional protection device. For example, an HARQ entity of the transmitting side may receive a NACK although an HARQ entity of the receiving side has transmitted an ACK or the HARQ entity of the transmitting side may receive an ACK although the HARQ entity of the receiving side has transmitted a NACK. Errors occurring during ACK/NACK transmission may cause a reduction in system performance. For example, if the transmitting side receives an ACK although the receiving side has transmitted a NACK, then the transmitting side will start transmitting a new packet. In this case, the receiving side cannot accurately reconstruct data since it receives a new transmission packet while awaiting receipt of a retransmission packet. In this case, data transmission delays may also occur since the upper layer of the receiving side will not request transmission of upper layer data from the upper layer of the transmitting side.